624 The P-N bond distance found in 1 is quite comparable to those found in the six-coordinate systems below (1.9 1- 1.98 A) wherein the electronegative fluorines on the five-coordinate phosphorane moiety are effective in attracting the nitrogen lone pair to the acidic phosphorus.I4
J
RF,P-0 R=Ph. F
I
Acknowledgment. , J.G.V. thanks the National Science Foundation for generous grant support of this research. Supplementary Material Available: fractional coordinates, bond distances, bond angles, and structure factors (6 pages). O r d e r i n g information is given o n any current masthead page.
References and Notes Figure 1. Computer drawing o f the l-hydro-2,8,9-trioxa-l-phospha-5aza-tricyclo[3.3.3.0]undecanecation of 1. Only the P-H hydrogen and one o f the disordered conformations is shown.
group ambiguity since they were intermediate between centrosymmetric ( P n a m ) and noncentrosymmetric ( P n a 2 l ) in value and the presence of a heavy atom could easily give spurious results. Solution of the crystal structure was begun by routine application of direct methods via a multiple solution weighted tangent formula* scheme in the noncentrosymmetric space group Pna21. All of the nonhydrogen atoms were easily located in the first phased E synthesis and all hydrogens located in subsequent difference F s y n t h e ~ e s .Full-matrix ~ least-squares refinements proceeded uneventfully to a final cyrstallographic residual ( A ) of 6.2% for the 615 observed reflections and 183 variables. The geometry of this model was very poorly behaved with chemically identical bonds differing by five standard deviations. The molecular constitution was essentially.that shown in Figure 1. Refinement was then attempted in space group Pnam (alternate setting of P n m a ) . It became rapidly apparent that a model in which the molecule sat on a mirror plane would not refine and models in which C(3) and C(4) were disordered were tried. These refined satisfactorily to an R of 6.5% for the 135 variables used. Fractional coordinates, bond distances, bond angles, and observed and calculated structure factors for this model are given in the supplemental material to be found in the microfilm edition; see paragraph at end of paper regarding supplementary material. A drawing of this model is given in Figure 1. The well-defined P-N dative bond in 1 (1.986 A) IS ' undoubtedly responsible for the unusual proton abstracting property of the bicyclic precursor. The phosphorus atom is clearly trigonal bipyramidal with 0-P-0 angles of 1 20°, 0-P-N angles of 8 7 O , and a H-P-N angle of 172'. This near trigonal bipyramidal symmetry of the five electron pairs around the phosphorus atom in 1 contrasts the relative instability of this electron pair configuration in: PX3-NR3 adductsi0 and intermediates involved in certain reactions of trivalent phosphorus compounds.' The P-H bond distance in 1 (1.35 A) is about 0.1 8, shorter than the 1.41 to 1.45 realized in several phosphines and PH4+I2 and is also somewhat shorter than the sum of the covalent radii (1.38 A). It is interesting that although trigonal bipyramidal phosphoranes tend to orient the more electronegative substituents on the apices, molecular constraint appears to preclude this possibility in 1. The similarity of the structure of phosphatrane 1 to that of silatranes (RSi(OCH2CH2)3N)13 with which it is isoelectronic is noteworthy. Journal of the American Chemical Society
(1) D. Hellwinkle in "Oraanic PhosDhorus COmDOUfldS". G. M. KosoloDoff and L. Maier, Ed., Gley-lntersdience, New York, N.Y., 1972, pp 185340. K. Dimroth and A. Nurrenbach, Chem. Ber., 93, 1649 (1960). J. G. Verkade, T. J. Huttemann, M. K. Fung. and R. W. King, Inorg. Chem., 4 . 8 3 (1965). As will be reported in detail later, Ph3CBF4 (from which proton abstraction is more difficult) leads to the expected [Ph3CP(OCH2CH2)3N]BF4 salt. Relative to 85% H3P04. (a) W. McFarlane and R. F. M. Whne, Chem. Commun., 744 (1969); (b) G. A. Olah and C. W. McFarland, J. Org. Chem., 36, 1374 (1971); (c) L. J. Vande Griend, J. G. Verkade, J. F. M. Pennings, and ti. M. Buck. submitted for publication. L. J. Vande Griend and J. G. Verkade. Phosphorus, 3, 13 (1973). G. Germain. P. Main, and M. M. Woolfson, Acta Crystalbgr., Sect. B, 24, 274 (1970). The following library of crystallographic programs was used: C. R. Hubbard, C. 0. Quicksall, and R. A. Jacobson, "The Fast Fourier Algorithm and the Programs ALFF. ALFFDP, ALFFT and FRIEDEL", USAEC Report IS-2625, Iowa State University-Institute for Atomic Research, Ames, Iowa, 1971: W. R. Busing, K. 0. Martin, and H. A. Levy, "A Fortran Crystallographic Least Squares Program", USAEC Report ORNL-TM305, Oak Ridge National Laboratory, Oak Ridge, Tenn.. 1965: C. Johnson, "ORTEP, A Fortran Thermal-Ellipsoid Plot Program", US. Atomic Energy Commission Report ORNL-3794. Oak Ridge National Laboratory, Oak Ridge. Tenn., 1965. R. R. Holmes and R. P. Wagner, Inorg. Chem., 2, 384 (1963), and references -. .- - - therein .. -. - .... (a) W. E. McEwen, Top. phosphorus Chem., 2, 5 (1965): (b) D. Denney and R. Dileone, J. Am. Chem. SOC.,64, 4737 (1962). D. E. C. Corbridge, Top. phosphorus Chem., 3, 91 (1966). H. Burger, Angew. Chem., lnt. Ed. Engl., 12, 474 (1973). (14) K. P. John, R. Schmutzler, and W. S. Sheldrick, J. Chem. Soc., Dafion Trans., 1841, 2466 (1974). (15) Camille and Henry Dreyfus Foundation Teacher-Scholar Grant Awardee, 1972-1977, and Fellow of the Alfred P. Sloan Foundation, 19731975.
J. C. Clardy,I5D. S. Milbrath J. P. Springer, J. G . Verkade* Ames Laboratory of the ERDA and the Department of Chemistry, Iowa State University Ames, Iowa 5001 I Received September 25, 1975
Directive Effects in the Electrophilic Substitution of Deltahedral Boranes and Heteroboranes. Halogenation of 1-SBgH9. Unusually Long llB Spin-Lattice Relaxation Times Sir: There has been considerable interest in the stereochemistry of electrophilic substitution of closo(deltahedral) boranesl and heteroboranes, at least in part because many members of this class of molecules can be termed aromatic. For deltahedral heteroboranes like 1,2- and 1,7-C2BioH12 and 2,4-C2BsH7, ground-state charge distributions correlate nicely with the results of electrophilic h a l ~ g e n a t i o n . ~ . ~
/ 98.2 / January 21, 1976
625 Table I. "B NMR Spectra of Halothiadecaboranesn 6-I-1-SB9H,: -71.2, d ( ~ ) , J B =H 175; 5.1, d (4),JBH = 180; 16.8; d ( ~ ) , J B H= 135; 19.9,d (1),JBH = 135; 24.1, S (1) 10-I-1-SB9H,: -50.1, S (I); 6.7, d (4),JBH = 185; 16.2, d (4), JBH= 165 6-Br-l-SB9H,: -69.3,d ( ~ ) , J B H= 175; 3.0,d ( ~ ) , J B =H 175; 6.9, = 190; 10.0, S (1); 15.4, d ( ~ ) , J B =H 180; 21.3, d (1) d (~),JBH JBH= 195 10-Br-1-SB9H,: -65.6, S (1); 8.3, d (4),JBH = 175; 18.4, d (4), JBH= 150 6C1-1-SB,H8: -68.7, d ( ~ ) , J B H= 175; 2.0, d (2), J B H = 145; = 180; 14.6, d ( ~ ) , J B H = 170; 23.2, d 4.2, s (1); 7.0, d ( ~ ) , J B H (l),JBH = 160 10-C1-l-SB9H,: -73.4, S (1); 3.3, d (4),JBH = 185; 17.3, d (4), JBH= 160 6,10-12-l-SB9H,: -48.8, S (I); 5.5,d (4),JBH = 180; 13.6, d (2), J B H = 155; 15.7,d ( ~ ) , J B H = 180; 23.5, s (1) 6,10-Br2-1-SB,H,: - 6 4 , ~ ( 1 ) ; 5 . 3 , d (2),JBH= 1 9 0 ; 7 . 6 , d (2), JBH= 175 10.4, S (1); 14.8, d ( ~ ) , J B H = 180; 20.6, d ( ~ ) , J B H = 185
6
=The chemical shift data are given in parts per million relative to external Et,O.BF, (- values downfield) followed by a description of the signal (d = doublet, s = singlet), its relative intensity (in parentheses), and observed coupling constant given in hertz. Spectra were determined at 32.1 MHz in CDC1, solution.
Figure 1. Structure and numbering convention for the thiadecaborane framework.
When 1-SB9Hg is treated with Cl2 in the manner used for bromination and iodination, only one isomer of chloro- 1thiadecaborane is observed, 6-Cl- l-SB9H8, the 1b isomer In the case of the deltahedral borane anions B12H1z2- and not the ax isomer predicted on the basis of charge distribuB,oHlo2- electrophilic halogenation4 does not appear to correlate well with predictions for sequential sub~titution.~ tion! Furthermore, when heated to 200 OC, 6-cl-l-SB9Hs isomerizes to an approximate 1:l mixture of ax:lb isomers. However, interpretation of the results was complicated We have also observed that after 20 h in a sealed tube at somewhat by the ease of halogenation and the difficulty of 200 OC the ratio of ax:lb Br-l-SB9Hs changes from 1:l to isomer s e ~ a r a t i o n .No ~ evidence for the rearrangement of 2: 1 and for I-l-SB9Hs the ratio changes from 1.6: 1 to 2: 1, deltahedral boranes or heteroboranes during the course of again indicating that the axial isomer is thermodynamically halogenation has been presented. We report here results on the halogenation of 1 -SB9H9 which can be followed sequen- more stable. These observations suggest but do not prove that initial tially and thus provide a good probe for the theory of direcattack is at the lower belt and that rearrangement occurs tive effects in substitution reactions in deltahedral boranes. during the course of iodination and bromination. Certainly, The results suggest that cage rearrangement plays a role in the reaction temperature was high enough for rearrangethe ultimate stereochemistry of the products. We also find ment in the case of iodination and apparently either the that certain B sites in these halothiaboranes manifest unvigor of attack by bromine or the nature of the transition usually long spin-lattice relaxation times ( T I ) .Although state results in the formation of considerable axial isomer our preliminary assessment indicates that the phenomenon during bromination. is isoIated to rather specific B sites, this communication is The course of sequential halogenation also appears to be in part a caveat regarding the determination of IIB N M R complicated by rearrangement. Our EHMO calculaspectra, a iask not usually plagued with difficulties due to tionss*i2suggest that both ax and lb isomers of CI-l-SBgHs saturation effects since relaxation is dominated by the very should yield 6,1 O-Cl2- 1-SBgH7 exclusively since for 6-CI- 1efficient quadrupolar m e ~ h a n i s m . ~ . ~ In keeping with previous observations for c a r b ~ r a n e s , ~ , ~SB9Hs the axial site is most negative and for 1O-Cl-lSB9Hs the lb sites are most negative. Indeed, the only dione would expect that those B sites furthest removed from bromo and diiodo isomer observed in the reaction mixtures the sulfur heteroatom would be most negative and most susis the 6,lO-isomer. However, no axial substitution has been ceptible to electrophilic substitution. This assumes that the detected in the case of the C12-1-SB9Hs which appears to be course of substitution is controlled by ground-state charge a mixture of Ib isomers as isolated from the reaction mixdistribution and is also not subject to rearrangement of the ture. framework. Our EHMO calculations corroborate this exStructural assignment is based on I i B N M R spectroscopecfation and give the following atom charges from a Mulpy (Table I). The structure of 2,2'-( 1-SBgHs)l is knowni0 liken population analysk8 upper belt (ub) B(2,3,4,5) = and allows the two doublets of relative intensity 4 in the I i B 0.2524; lower belt (lb) B(6,7,8,9) = 0.0733; axial (ax) spectrum of 1-SB9Hsi4to be assigned. The lower-field douB(10) = -0.0734 (see Figure 1 for atom numbering conblet is due to the belt of four borons adjacent to the sulfur vention). (ub) and the higher field doublet to the other belt of four When 1-SB9H9 is treated with an equal amount of either I2(200 OC) or Br2 (25 "C) in a sealed tube (with or without borons (lb). In general for lb monosubstitution, the N M R aluminum halide) a mixture of lO-X-l-SB& and 6-X-lsignals for the unsubstituted ub borons displayed relatively SB9Hs results." The ratio of ax:lb substitution varies belittle change from the corresponding region for l-SBsH9, but major shifts were evident in the Ib region. In all cases, tween about 2:l and 1:l depending on the run even though the barycenter for the ub and Ib regions was maintained there is a four times greater statistical chance for lb vs. ax within f l and f 3 ppm, respectively, of the corresponding substitution. At first glance these results appear to be in acsignal for I-SBsHg. The ax boron resonance is quite far cord with ground-state charge distributions; however, we downfield from the other signals and appeared as a doublet suspect differently in view of the results of chlorination and of relative intensity 1 for all lb-substituted products. For the isomerization experiments. Communications to the Editor
626 ax isomers, the IIB N M R spectra were quite simple and showed the expected downfield singletI5 and two upfield doublets of relative intensity 1:4:4. However, as previously alluded to, the singlet was unusually hard to detect. In the case of IO-X-l-SB9H8 (X = C1, Br, I) we observe only two doublets of equal intensity when the IlB N M R spectrum is determined with a short delay (10-100 ms) between ir/2 pulses. The expected low-field singlet for the halogenated B site gradually grows in as the delay time is increased above 100 ms and reaches a limiting intensity (1 relative to 4 for the two doublets) at 8-10 s.16 The intensity recovery for the singlet as the delay time increases follows first-order kinetics and was used to estimate17 T I = 2.8, 3.9, and 2.6 s for X = C1, Br, I, respectively. The ir, 7,*/2 pulse sequence method of VoldI8 was used to determine the much shorter T I ' Sfor the other B environments (ub = 10, 14, 8 ms; and Ib = 24, 22, 19 ms for X = C1, Br, I, respectively. For comparison, the Ti's for 1-SBgHg are ax = 34, ub = 21, and Ib = 40 ms, respectively. The halogen has a decided effect on the relaxation time of the axial B site. This seems even more remarkable in view of the T I results when the halogen is in a Ib position, such as for 6-Br-1SBgH8 where T I = 35 ms for the substituted boron and ranges from 11 to 22 ms for the other positions. We do not feel that this effect is related to anisotropic molecular reorientation. For symmetric rotors, the relaxation time of a nucleus on the symmetry axis (such as the axial B site of I-SBgHg) depends only on motion perpendicular to that axis.I9 As estimated from moments of inertia, such motion would give rise to longer correlation times (shorter T I ' S )for lO-X-I-S&& (X = c1, Br, I) in comparison with 1-SBsHg. Moreover, for 6, IO-Br2- l-SB9H7, where the symmetric-top nature of lO-Br-I-SB9Hg has been removed by substitution of a second halogen on the Ib, the axand Ib-substituted borons still manifest the same large disparity in T I values. The antipodal position of sulfur may somewhat influence T I ;however, we have found no saturation effects when running the spectrum of 12-Br-l-SB11Hl0, the icosahedral closo thiaborane with bromine substituted antipodal to the sulfur.20 One perhaps salient difference between the axial positions in 1-SBgH9 and I-SBIIHII is the coordination number of the deltahedral vertex, five in 1-SBgHg and six in 1-SBI I H I 1. However, we have no other axially halogenated 1-heterodecaboranes for comparison ((ax-I-Bl0H9)~-shows no saturation problemsZ1but perhaps the unreported (10X- 1-PB9H8)- or (10-X- 1-CBgHg)- would). A comparison of T I values for l"B and "B led Allerhand, Odom, and Moll to conclude that all contributions other than quadrupolar were negligible for B2H6, B5H9, and B(C2H5)3; it was also estimated that electric field gradients at boron are relatively smalL6 It is our estimation that the electric field gradient is accidentally very small at the axial B site in lO-X-l-SB9Hs molecules even though this is not corroborated by EMHO calculations.22 In the absence of effective quadrupolar relaxation, it is entirely possible that scalar relaxation, caused by spin-spin interaction of the axial boron and the attached halogen, is dominant here. Since our present experiments only show t ; ~ a tT2 (effective)
